The present invention relates to the field of acoustics and, more particularly, to the processing of audio signals to provide control over the cross-correlation of a pair of audio output signals.
The interaural cross-correlation of the signals reaching the ears of a listener has long been recognized as an important acoustic predictor of subjective sound properties. It is especially relevant for concert halls, for which a low interaural cross-correlation gives rise to the highly desired sound quality of "spaciousness"[Schroeder, M. R., Gottlob, D., and Siebrasse, K. F., "Comparative study of European Concert Halls: Correlation of Subjective Preference with Geometric and Acoustic Parameters", Journal of the Acoustical Society of America 56, pp. 1195-1201 (1974); Ando, Y., "Subjective Preference in Relation to Objective Parameters of Music Sound Fields with a Single Echo", Journal of the Acoustical Society of America 62, pp 1436-1441, (1977)]. It has also been demonstrated that the cross-correlation coefficient of two noise signals presented to listeners was strongly correlated with the perceptual width and distance of the acoustical image [Kurozumi, K. and Ohgushi, K., "The Relationship Between the Cross-correlation Coefficient of Two-channel Acoustic Signals and Sound image Quality", Journal of the Acoustical Society of America 74, pp. 1728- 1733 (1983)]. Image distance is directly correlated with the value of the cross-correlation coefficient, and image width is inversely correlated to the absolute value of the cross-correlation coefficient. These authors have also shown that the absolute effect of cross-correlation coefficient is greater for low frequencies (below 1KHz) than for high frequencies (above 3Khz).
The cross-correlation of two signals, y.sub.1 (t) and y.sub.2 (t), is typically measured in terms of a cross-correlation measure which is defined to be the extreme value of the cross-correlation function .OMEGA.(x), where ##EQU1## The cross-correlation measure has a maximum possible value of 1 and a minimum possible value of -1.
The cross-correlation measure of the output signals of an apparatus will typically be very close to the interaural cross-correlation of the signals reaching the ears of the listener when sound is produced by loudspeakers or headphones. The actual interaural cross-correlation will be somewhat dependent on the characteristics of the reproduction environment. For example, room reverberation will tend to shift the interaural cross-correlation toward zero.
Prior art systems which produce acoustical effects and manipulate the cross-correlation measure are known to those skilled in the art. For example, such systems have been used to broaden the image of stereophonic input signals.
Shimada (U.S. Pat. No. 3,892,624) and Doi, et al. (U.S. Pat. No. 4,069,394) describe a stereophonic reproduction system in which portions of the input signals are scaled by a constant, k, and cross-fed in 180-degree out-of-phase relationships. That is, given left and right input signals a.sub.1 (t) and a.sub.r (t), left and right output signals L=a.sub.l (t)-ka.sub.r (t) and R=a.sub.r (t) are generated. When L and R are presented over two loudspeakers, a listener located between the loudspeakers perceives a broadened sound image.
Cohn (U.S. Pat. No. 4,355,203) teaches a method for providing signal decorrelation in which a time delay is utilized. In this system L=a.sub.1 (t)-ka.sub.r (t-T.sub.d) and R=a.sub.r (t)-ka.sub.1 (t-T.sub.d), where T.sub.d is the time delay in question.
The above mentioned systems and systems based on similar techniques all manipulate the cross-correlation of the output signals. It should be noted, however that the authors of these references do not characterize the operation of their various apparatuses as cross-correlation measure manipulation apparatuses.
These prior art methods for manipulating the cross-correlation measure have a number of problems. For example, consider the case of a single sound element (such as a monophonic track from a mixing console or tape recorder) shared by the stereo input channels in some ratio, L:R. The cross-correlation measure at the output channels will be either positive one or negative one depending on the L:R ratio and the relative gain, k, of the cross-fed, out-of-phase signals. Input signals which contain a multiplicity of such single sound elements produce an output which can be viewed as a strict summation of the output of each single sound element. Given that these systems are designed to process input signals with multiple sound elements (each with its own L:R ratio), the final result is greatly dependent on the program material. Furthermore, center images are less intense than side images. When the L:R ratio of the program material is equal to one, a.sub.1 (t) equals a.sub.r (t) and the subtraction of signals in each channel results in a loss of intensity in each output. Hence, these systems do not work well for all types of program material.
Furthermore, the range of cross-correlation measure values that can be generated utilizing these techniques is restricted to a small range of the possible cross-correlation measure values. It can be shown that cross-correlation measure values outside the ranges produced by these techniques may be advantageously utilized to provide acoustical effects.
Another problem with these types of systems is the colorization added to the final output signal. The summation of the signals used to provide the output signals results in constructive and destructive interference. This interference alters the perceived timbre of the sound. In addition, the interaural phase relationship at the listener's ears is highly dependent on the listener's location relative to the loudspeakers and causes listeners at these locations to hear quite different effects in timbre, image width, and image distance.
Another type of system that manipulates the cross-correlation of the output signals is taught by Orban (U.S. Pat. No. 3,670,106). The apparatus taught by Orban is utilized in converting a monophonic sound signal to stereophonic sound signals. In this system, the monophonic sound signal is processed with an all-pass filter to form a second signal with an added phase shift. The phase shift in question varies slowly as a function of the frequency of the monophonic signal. The second signal is then added to and subtracted from the original monophonic sound signal to produce left and right stereophonic speaker signals, respectively.
These left and right speaker signals are the result of the constructive and destructive interference of the original monophonic signal with the second, all-pass filtered signal. The phase of the all-pass processed signal determines the magnitude and phase response of the output signals. A comparison of the magnitude response of the output signals across frequency reveals that when the left magnitude response is at a maximum, the right magnitude response is at a minimum and vice versa. This helps to reduce the timbral coloration. A comparison of the phase response also reveals a similar complementary relationship. Therefore, it can be seen that this system uses both inter-channel amplitude and phase differences to steer the sound image from side to side. The effect of the system is achieved primarily through differences in the magnitude of the channels rather than through phase differences. The author points out that "very slight phase shifts" are utilized. Viewed from the standpoint of the psychoacoustic phenomenon of time-intensity trading, the large magnitude differences (.infin.dB at "cross-over frequencies") overwhelm the impact of the slight inter-channel phase differences (approximately .pi./10 in the preferred embodiment).
A "third control element" is mentioned which adjusts "the channel separation from pure, completely in-phase monophonic to pure, random phase stereo." In regards to the "random phase stereo", this statement is neither supported nor is it true. The phase shifts created by this system in the individual output signals are not random but occur in a repeated pattern centered at each of the predetermined "cross-over points." Then too, magnitude differences are dominating the phase differences.
One problem with this system is that the complementary maxima and minima of the magnitude response cause coloration for a listener located closer to one loudspeaker than the other.
Furthermore, the range of cross-correlation measure values that can be generated utilizing this system is restricted to a small range of the possible values. It can be shown that cross-correlation values outside the range provided by this system may be advantageously utilized to provide acoustical effects.
Although this system creates the illusion of a broadened sound image, the image in question is less than ideal. The slow variation of the phase shift with frequency results in the image appearing to be "broken". That is, different frequency components of the image are located at the locations of the different speakers. For example, the sound in the broad frequency band about 500 Hz might appear to emanate from the left speaker, while the sound in the frequency band about 1000 Hz appears to emanate from the right speaker, the sound in the frequency band about 2000 appears to emanate from the left speaker, and so on. This is the result of frequency banding which is imposed by requiring the added phase shift to vary slowly with frequency.
Broadly, it is an object of the present invention to provide an improved apparatus and method for controlling the cross-correlation measure of any two output signals.
It is another object of the present invention to provide an apparatus and method for controlling the cross-correlation measure of two output signals which is capable of producing cross-correlation measures over the full range of possible values.
It is yet another object of the present invention to provide an apparatus and method for controlling the cross-correlation measure of two outputs signals which does not alter the color of the sound.
It is a still further object of the present invention to provide an apparatus and method for controlling the cross-correlation measure of two output signals which does not depend on the program material.
It is yet another object of the present invention to provide a sound broadening apparatus and method which does not produce a sound image which appears to be spatially broken.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.